The present invention relates to thermal printing techniques and, more particularly, to a method for improving the crispness of the thermal printing image, with the reduction and elimination of halo effects.
Two-color, direct, thermal printing is a process in which combinations of colors, such as red and black, are imaged onto a coated paper. In the area of two-color printing, most of the printing is text based. The printing is controlled by an algorithm. The algorithm must be carefully coordinated with the design of the print head and the type of paper being used, in order to achieve an optimal image.
The method of this invention seeks to improve the image quality of graphics and other high resolution data.
The method of the invention uses a thermal printing paper type that is designed for red and black. Red dye in the paper has an image temperature that is distinct from the image temperature for black dye in the paper. Increasing the applied energy to the print head therefore causes a transition from red to black. Moreover, careful control of the dot-to-dot energy on the thermal print head provides distinct red and black color shades.
In two-color printing systems, there is a phenomenon commonly known as the xe2x80x9chalo effectxe2x80x9d. The halo effect is a bleeding of the lower energy color (red), after the higher energy color (black) has been printed. At the termination of the higher energy level, the black dots being printed develop a surrounding shade of red as the printing temperature decays. This decay, or xe2x80x9cthermal fallxe2x80x9d, creates the red border or halo about the black image. The magnitude of the halo is proportional to the thermal fall time.
The halo effect is known to be less perceivable when higher concentrations of black are used. However, the halo is not related to the amount of black being imaged. It is related only to the thermal fall time of the print head. The thermal fall time is constant, and is a function of the print head design. The result is that large black squares appear black, while smaller black squares appear maroon, or a combination of red and black.
Typically, real-time control (i.e., dot-history) can be achieved only in limited sections during printing. This is sufficient for text, but complex graphics, such as logos, coupons, etc., require additional preprocessing.
It is possible to preprocess an image in order to filter out special combinations of black dots and black dot patterns. These patterns can be replaced with printer-friendly patterns without significantly altering the shape of the image form. This pre-filtering method, which is essentially an off-line dot-history control, greatly reduces the associated halo effects. Pre-filtering of an image can be achieved using a printer or a host personal computer. The use of pre-filtering:
a) reduces the demand upon the printer or firmware,
b) can be applied to more combinations for which real-time dot-history can compensate, and
c) can be applied to the entire image.
The result can be previewed before printing, when applied upon a host personal computer.
A series of special algorithms has been developed, in accordance with this invention, for computing and adjusting an image to provide the best clarity under a certain set of constraints. These constraints are based upon the dot-history control and the thermal properties of the print head.
In accordance with the present invention, there is provided a method for improving a thermal print image. A series of special algorithms has been developed for computing and adjusting the thermal print image to provide the best clarity under a certain set of constraints. These constraints are based upon the dot-history control and the thermal properties of the print head. An image is preprocessed using image and color matrices, in order to filter out special combinations of black dots and black dot patterns. These patterns are then replaced with printer-friendly patterns. This altering technique improves the image quality, does not significantly alter the shape of the image form, and greatly reduces associated halo effects. Cell areas having a cell size of 4xc3x971 or less are the maximum processing area that can be achieved without deviating from the original image. A 4xc3x972 cell can be processed by horizontally copying a 4xc3x971 cell into its adjacent neighbor.
It is an object of this invention to provide a method for improving a thermal print image.
It is another object of the invention to provide a method for improving a thermal print image quality by filtering out certain black dots and black dot patterns of the thermal print image, and thereafter replacing these patterns with printer-friendly patterns.